1. Field of the Invention
The invention relates generally to beam-forming antenna systems and more specifically to microwave reflector type antennas with horn illuminators.
2. Description of the Prior Art
The combination of a parabolic or partially parabolic reflector illuminated by a horn antenna is one of the earliest antenna system arrangements employed in radar systems for the generation of a highly directive beam in space, and accordingly is extensively described in the technical literature. The text "Antenna Engineering Handbook," Henry Jasik, Editor (McGraw-Hill 1961) provides an overview of the art in that respect in Chapter 25, section 25.2, entitled "Radar Antennas," and also describes the state of the art in respect to horn antennas per se in its Chapter 10. From these descriptions and their extensive bibliographical references, a quite complete background in the pertinent prior art in the environment of the invention is obtainable.
For ultralow-sidelobe performance in a reflector antenna with horn feed, the precise pattern shape of the feed horn is very important. The near-in (in angle) sidelobe is very sensitive to the pattern shape of the feed horn from the beam peak (0 dB) to approximately -10 dB. At larger angles from the said horn access (line of boresite), the sidelobe level is likely to be dominated by the "spillover lobe." Although such a spillover lobe can be reduced by increasing the size of the reflector, such an approach may not be acceptable because of size and weight constraints.
The manner in which the invention controls the pattern of the horn feed itself so that it drops off rapidly as a function of increasing angle from the horn axis, while also providing a desirable reflector illumination distribution, will be understood as this description proceeds.